In recent years, research and development work has been briskly carried out on wireless communication technologies that utilize millimeter waves of frequencies greater than or equal to 30 GHz as high frequency signals. The wireless communication technologies utilizing millimeter waves as high frequency signals have been adopted for data communications and radars. High frequency substrates for use in wireless communications are required to have excellent transmission characteristics.
As typical transmission lines for transmitting high frequency signals such as millimeter waves, there is known a laminated waveguide in which a pseudo waveguide is formed of through conductors and electrically conductive layers in a multilayer circuit board. When it is desired to construct the laminated waveguide at a high level of integration in area, there may arise a need to turn the direction of transmission of high frequency signals from a planar direction to a thickness-wise direction. However, if the transmission direction in the laminated waveguide is turned to the thickness-wise direction, reflection of high frequency signals will take place at the turn of transmission direction, thus causing significant transmission loss. As a result, the transmission characteristics of the laminated waveguide may be deteriorated considerably.
Where transmission lines employing a rectangular waveguide are concerned, in Japanese Unexamined Patent Publication JP-A 9-199901 (1997), there is disclosed a technology of imparting a turned-back configuration to a transmission line by using a folded-waveguide. However, even if this folded-waveguide technology is applied to formation of a laminated waveguide, considerable deterioration in transmission characteristics of the laminated waveguide is inevitable.
An object of the invention is to provide a waveguide structure having excellent transmission characteristics, and a high frequency module including the waveguide structure, and a radar apparatus.